In a number of analytical techinques to determine the chemical constitution of solids, charged secondary particles are observed. In Auger analysis and ESCA (Electron Scattering for Chemical Analysis) the secondary particles are electrons emitted from a surface subjected to a high energy electron beam or an X-Ray beam. The energies of the secondary electrons being characteristic of the chemical elements in the surface. Known analyzers measure these energies.
In Secondary Ion Mass Spectrometry (SIMS), the detected secondary products are either positively or negatively charged ions, the chemical constitution of which is readily determined by passing the ions through a mass-to-charge analyzer or mass spectrometer. However, the secondary ion energy distribution is large, extending to several tens of electron-volts; and mass-to-charge analyzers need a fairly limited range of energies for good mass resolution. To provide same, an energy filter serves the purpose of selecting from the total distribution a limited part of the energy distribution for transmission of same through the mass-to-charge analyzer.
A number of devices are commonly used to accomplish both these tasks. Those known as energy analyzers are designed to transmit a very narrow band of energies; however, when used as energy filters, the band of energies transmitted by the device is enlarged, usually in the interest of increasing the charged particle current which is the signal observed. Such devices are described in many textbooks (See for example, Poul Dahl, "Introduction to Electron and Ion Optics," Academic Press, New York, 1973, Chapter 6.), and include parallel plate analyzers, 127.degree. cylindrical analyzers, 180.degree. spherical analyzers, and cylindrical mirror analyzers, all of which are well known to those skilled in the art of charged particle energy analysis. These devices use electrostatic fields and focus ions of the same energy but initially moving in different directions to the same area or line, while focusing ions of different energies to different areas or lines. Slits or apertures are placed at the location of the area or line to which particles of the desired energy are focused so that these particles emerge from the device while particles of other than the desired energy do not.
A second type of energy analysis system is the Retarding Potential Difference method (RPD) first reported by R. E. Fox, W. M. Hickam, T. Kjeldas and D. J. Grove in the Physical Review, Volume 84, page 859 (1951). In this method the charged particles are induced to travel in a straight line and pass through several apertured plates or planar grids. On one of the plates a sufficient barrier potential is established to repulse particles with less than predetermined energy so that such particles are repulsed, while particles with more than the minimum energy pass through the barrier and effectively are transmitted. The distribution of energies is obtained or the energy analysis is performed by slightly changing the barrier potential and observing the change of transmitted charged particle current resulting therefrom. Particles are not deflected in the RPD method (except for those which are deflected through 180.degree. in being repulsed or mirrored backward) and the RPD analyzer is thus basically a low energy cut-off device.
Recently another class of energy analyzers have been introduced which are known under the general name of "Bessel Boxes", a name deriving from the mathematical solution of the potentials within these analyzers wherein it is necessary to use a series of Bessel Functions.
In these analyzers charged particles are admitted through an aperture (or annulus) in a conducting entrance plate, the plane of which is perpendicular to the axis of and adjacent to one end of a hollow conducting main cylinder. A conducting exit plate containing an aperture (or annulus) is situated at the opposite end of the cylinder so that the exit plate is parallel to the entrance plate. Potentials are placed on the entrance and exit plates and on the main cylinder whereby charged particles are retarded after entering the device in a manner reminiscent of the RPD method. And, as in the RPD method, if a particle does not have a minimum energy as determined by the potentials, it will be repulsed back toward the entrance aperture. However, here the analogy ends. Particles in a Bessel Box are not restricted to straight line movement along the axis of the device; the structure is such that they cannot move along the axis of the device. Where annuli are used in the end plates, particles enter the box off its axis; where apertures are provided at the ends to admit on the entrance end the particles moving in the direction of the axis, a solid stop, disposed in the center of the main cylinder, intercepts any particles which may be moving along the axis. It is thus necessary, one way or another, for a particle to be deflected if it passes through a Bessel box. A particle with energy which is slightly more than the minimum requisite energy moves relatively slowly through the main cylinder and is substantially deflected by the weak electric fields therein into the exit aperture. However, particles having energy which is substantially higher than the minimum are deflected insufficiently by the weak fields to arrive at the exit aperture. Thus a Bessel Box possesses a high-energy as well as a low-energy cut-off, unlike the RPD devices which possess only the low energy cut-off.
A Bessel Box is described by J. D. Allen, Jr., J. D. Durham, G. K. Schweitzer and W. E. Deeds in the Journal of Electron Spectroscopy and Related Phenomena, Volume 8, pages 395-210, (1976) provides annuli at both the exit and entrance apertures without utilizing electrodes or other elements within the Bessel Box, as such. This instrument was found to give very good energy resolution and has been employed as an energy analyzer. Another Bessel Box of which the inventor has knowledge, used as an analyzer with good energy resolution, is one which insofar as known has not yet been described in scientific or patent literature, but has been recently used by Mr. Peter Erdman and Professor Edward C. Zipf of the Department of Physics at the University of Pittsburgh. It has small circular apertures placed exactly in the center of both the entrance and exit plates. Particles are prevented by going on a straight line path between apertures by affixing within the box a circular metallic stop, having a diameter slightly larger than the diameter of the apertures, exactly in the center of the device.
Both Bessel Boxes of Allen and his associates and of Erdman and Zipf, which are used as energy analyzers, require very narrow annuli or small apertures. Although they achieve good energy resolution, the signals of transmitted currents are small.
The inventor has found that by substantially increasing the aperture diameters and annuli gaps, the acceptance apertures of Bessel Boxes are increased and sufficient energy resolution is maintained to permit their use as efficient energy filters, apposite energy analyzers. In fact these modified Bessel Boxes actually perform more satisfactorily than the conventional energy filters particularly with regard to: () the efficienty of the acceptance of the aperture, and (b) the independence of establishing an energy pass band width from the mean energy of particles transmitted. Moreover, these modified Bessel Boxes are particularly useful as energy filters in combination with quadrupole mass spectrometers as detectors in secondary ion mass spectrometry.